The present invention relates to a magnetic recording medium, and more particularly to a perpendicular magnetic recording medium which includes a magnetic recording layer having an axis of easy magnetization in the direction essentially perpendicular to its own layer surface.
With advancements in information processing techniques, the quantity of information handled by memory devices has increased rapidly in recent years. There is a corresponding increased demand for greater data storage capacity in magnetic recording media such as floppy disks. In response to this demand, active research and development have been pursued on magnetic recording media with high density capability, in particular on perpendicular recording media which realize magnetic data storage by generating residual magnetism in the direction essentially perpendicular to the layer surface. The magnetic recording layer has a perpendicular axis of easy magnetization in a direction essentially perpendicular to the layer surface of its own.
As such a magnetic recording layer, there is known a metallic film type medium formed by sputtering or vapor deposition of a metallic film such as a Co-Cr system alloy or the like, or an oxide film type medium having a magnetioplumbite type crystal structure as Ba ferrite or Sr ferrite. Such a perpendicular magnetic recording medium is considered to be promising as a medium more suitable for high density recording than a spreading type medium that is used for the majority of the longitudinal magnetic recording media.
In a spreading type medium, the magnetic recording medium is formed by spreading magnetic powder on a base by mixing it with a binder or the like. Therefore, the magnetic recording layer formed is elastic, and in addition, it is possible to mix a lubricant in the magnetic layer. This leads to maintaining a satisfactory contact between the magnetic medium and the magnetic head, making it possible to give a sufficient durability to the medium and the head.
In contrast to this, a magnetic recording layer of the metallic film type medium or the oxide film type medium has little elasticity so that, when a magnetic head made of a hard material such as ferrite runs on a magnetic medium, damages such as scratches tend to be created on the surface of the medium or the head. In that case, not only the durability of the medium and the head is injured, but also the effective spacing between the medium and the head is increased due to adherence of the powder abraded from the medium or the head. This results in an increase in the spacing loss which causes degradation in the frequency characteristic and lowering and variations in output power during the reproducing (reading from the disk).
For this reason, it is considered, in the case of the metallic film type and the oxide film type medium, to form a protective layer made of a hard material over the magnetic recording medium in order to protect it. As a concrete example of the protective layer, there has been proposed a film of silicon oxide, aluminum oxide, titanium nitride, and others. However, the quality of material of such a protective layer is brittle so that it tends to be worn out by its contact with the magnetic head. Then, the resulting worn powder damages the medium and the head, showing that it will not be sufficient as a solution to the above problem.
Forming the protective layer thick enough to be effecting for preventing abrasion of the magnetic recording layer, would make it undesirable from the viewpoint of the perpendicular magnetic recording characteristic. Namely, the recording density of the perpendicular magnetic recording system is essentially much higher than that of the longitudinal magnetic recording system, and permits the recording wavelength to be reduced. However, in order to accomplish these it becomes necessary to restrict the effective spacing between the head and the medium to an extremely small value, and accordingly, the thickness of the protective layer is limited also. When the thickness of the protective layer is restricted in this way to the extent that it does not injure the perpendicular magnetic recording characteristic, the effects on the improvement in the durability of the layer may not be expected too much.
On the other hand, as an alternative there may be considered a method of spreading a lubricant over the magnetic recording layer. However, in the case of a metallic film type medium, the film formed by sputtering or a like method has such a smooth surface that the wetting and holding power for a lubricant are low. Therefore, it is difficult to spread a lubricant over the layer with sufficient adhesive power and uniformity so that the above problem still remains unsolved.